North Cascades Geology

The Work of Running Water

Skagit River near Newhalem

To understand the role of stream and river erosion
in shaping the North Cascades, geologists look
to the events creating the major drainages of
the region such as the Columbia River, which cuts
across the present day Cascade Range. The river’s immediate ancestor
probably developed from streams running off an
elevated block of crust in Washington and British
Columbia dating from the uplift of land underlain
by the thrust-thickened crust, probably sometime
between 90 and 50 million years ago (middle Cretaceous
to early Tertiary).
Tributaries to the Columbia River and other lesser
rivers to the north began etching out the ancestral North Cascade mountains,
probably in
a linear trellis pattern reflecting the major
northwest-southeast alignment of the major rock units. (see How
the Rivers Work)

Not long after (geologically speaking, of course)
the ancestral Columbia River was established,
probably about 60 million years ago (in the earliest Tertiary),
tensional forces in the crust associated with the Eocene extensional event broke
the region into fault blocks. Some blocks rose high enough to become
mountains, some sunk sank low to become basins. Streams began to fill
the basins with sediments mostly eroded from nearby highlands. Professor
J. Hoover Mackin, an ardent fan of Pacific Northwest geology, imagined
a scene not unlike the Basin and Range region of Nevada and Utah today,
with upraised fault block mountains, exposing old rocks, surrounded
by alluvial basins, slowly filling with sand and mud. Mackin called
this collection of sunken basins and uplifted blocks the Calkins Range,
after Frank Calkins, an early
North Cascades geologist.

View east across the Chiwaukum Graben, a down-dropped block of river-deposited sandstone and conglomerate, which first formed as a structure in the Calkins Range. Visible strata in the graben shows the folded beds of sandstone. The more easily eroded sedimentary rocks form the the low hills betweeen higher ridges of resistant gneiss and schist such as seen here in the higher Entiat Mountains. The west-bounding fault is hidden behind the foreground ridge of schist.

Although much of the material deposited in the basins came from nearby highlands, some of the sand grains in the sediment came from farther east, supporting the idea that rivers, like the Columbia, had already established their courses through the uplifted terrain. Not only were basins forming as the crust was stretching, but blocks of crust to the west were generally moving north relative to blocks to the east, along major faults such as the Straight Creek Fault.
Volcanoes must have erupted above some of the plutons associated with
the Eocene extensional event, and some of the stream patterns of the
North Cascades may well be relicts from rivers draining radially off
these volcanoes.

Whatever drainage pattern was established, it
was profoundly altered about 35 million years
ago (Oligocene) by the renewed volcanic
activity of the Cascade Volcanic Arc.
Although only remnants of these ancient volcanoes exist here and
there in the North Cascades, the whole range during this period
was probably blanketed by lavas and breccias, just as it still is
today in southern Washington.

Preservation of volcanic cover in southern Washington.

The growth of these volcanoes must have diverted
numerous steams and established a mostly entirely new drainage system,
but one reflected in the position of the major drainage divide today.
This divide extends roughly north-south through North Cascades National
Park, from Mt Redoubt on the north to Damnation Peak on the south, and
separates streams that flow west into the Chilliwack, Baker, and lower
Skagit Rivers from east-flowing streams that join the upper Skagit.
This pattern may have been established as streams flowed down the slopes
of the arc volcanoes that once erupted above the Chilliwack
Batholith. However, as this volcanic cover was eroded off, differential
erosion worked its magic in readjusting the streams and rivers to
the hardness and softness of the rocks beneath the volcanic cover. Much
of the drainage may have found itself back in the old patterns it had
before the volcanic interruption. Creeks once again widened fault valleys,
and hard metamorphic and granitic rocks were left as lofty crags.

As the range continued to rise, the volcanic rocks were stripped away, except in places where they had been faulted down into the older rocks and are preserved still. Some are preserved because they were metamorphosed to hard rock by nearby intrusion of arc-root plutons. Mount Spickard and Ruth Mountain are examples of rugged peaks underlain by recrystallized (hornfelsic)
volcanic rocks.

The overall drainage pattern we see today, with a
few spectacular exceptions, was well-established
by the time extensive glaciers began to form about 1.6 million years
ago (Pleistocene). Pleistocene glaciers,
however, put the final touches on almost every scene. The evidence of
glacier erosion or deposition is everywhere.

Material in this site has been adapted from a book,
Geology of the North Cascades: A Mountain Mosaic by R. Tabor
and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published
by The Mountaineers, Seattle.